Eshell: Mostly cosmetic changes to be more explicit about dynbind

Eshell: Mostly cosmetic changes to be more explicit about dynbind

* lisp/eshell/em-dirs.el (eshell-dirs-initialize): Reveal the lambdas.
(eshell-expand-multiple-dots): Fix ^+$ => \`+\' in the regexp.

* lisp/eshell/esh-cmd.el (eshell-this-command-hook): Declare as
dynamically scoped.
(eshell-trap-errors): Use `mapc funcall` since this can't have
global/local settings like a true hook.
(eshell-do-eval): Split the `let` case from the rest so we can use
`cl-progv` rather than `eval` for it.
(eshell/which): Use `fboundp` test instead of ugly
gymnastics to try and hide the function call from the compiler.

* lisp/eshell/esh-var.el (eshell-variable-aliases-list): Reveal the lambdas.
(eshell-parse-variable-ref): Use backquotes.